Display elements using nematic liquid crystals have several modes of aligning liquid crystal molecules. The twisted nematic (TN) mode is most widely used, but there are other modes such as the birefringence modes with a homeotropic (vertical) alignment or a homogenous (plane) alignment, and the guest host LC mode etc.
The TN liquid crystals are in a stable condition when nematic liquid crystals provided with positive dielectric anisotropy are placed between substrates which are planely aligned and disposed with electrodes, and a liquid crystal molecular long axis is successively twisted between the substrates at 90.degree.. In this case, linearly polarized light enters vertically to the substrate, and a polarization plane of the linearly polarized light is rotated along the alignment of liquid crystals at 90.degree.. Therefore, when polarizers and analyzers are disposed perpendicular to each other, the display becomes white. Furthermore, when liquid crystal molecules vertically align under applied voltage, incident linearly polarized light proceeds further into the liquid crystal layer, so the diplay becomes black due to absorption by the analyzers.
Since liquid crystal display elements in the TN mode or in the birefringence mode require a polarizer, the polarized light is absorbed in natural light. Therefore, transmissivity would not be more than 50% even under ideal conditions, and it is usually about 20 to 30%. As a result, particularly when reflective liquid crystals utilizing outer light are used, the display turns out to be extremely dark.
An example of a bright mode which does not use a polarizer is a phase change guest host LC mode shown in FIG. 4. At present, this mode is most intensively developed due to its brightness and high contrast. Furthermore, this mode is used to experimentally manufacture a reflective multicolor TFT-liquid crystal display panel (for example, S. Mitsui, Y. Shimada et al., SID'92, pp 437-440). By sandwiching guest host liquid crystals, which are made by mixing a dichroic dye and cholesteric liquid crystals having a comparatively short twist pitch between substrates, twist spirals are aligned in the vertical direction to the substrate.
In this instance, incident light is absorbed into the dye, so that when, for example, black dye is used, the display would be black. The dichroic dye has an absorbance axis along a long axis direction of liquid crystal molecules, so that absorbance becomes higher when the incident linearly polarized light is not rotated optically by the liquid crystal molecules. Therefore, host liquid crystals having as small birefringence index (.DELTA.n) as possible are used. When voltage is applied to these liquid crystals, the screw axes first become horizontal to the substrate as shown in FIG. 4 (b). When voltage is applied further, the twists become loose, and the vertical alignment shown in FIG. 4 (c) is attained. In this case, the absorbance of the dye is small, so that the color of the reflection plate on the backside looks brighter.
The guest host liquid crystals are characterized in that as a result of enlarging cell thickness d or increasing dye concentration, contrast increases but the brightness detereorates. A logarithmic ratio of transmissivity (or reflectance) between the dark condition and the bright condition is called a "dichroic ratio", which serves as an index of performance for the guest host liquid crystals. As the dichroic ratio becomes larger, it is possible to obtain a display with more brightness and higher contrast. In order to enhance the dichroic ratio in the phase change guest host LC mode, it is effective not only to decrease the biregringence index .DELTA.n, but also to increase the ratio d/p of the cell thickness d and the twist pitch p of the liquid crystals. However, the harmful effect of increasing d/p is that the driving voltage becomes proportionally higher.
A value of generally used d/p is about 2, and the driving voltage is as high as around 10 V, which is more than twice as much as that of the TN mode. Furthermore, threshold voltage at the time of raising the voltage and dropping the votage differs, so hysteresis results. Therefore, it is difficult to display half tone. In addition, the guest host liquid crystals are not suitable for matrix driving, since the intermediate condition of changing the direction of screw axes occurs between the condition of non-voltage and the condition of saturation.
An another mode which does not use a polarizer is a mode shown in FIG. 5, in which a quarter wavelength plate 31 and a reflection plate 32 are positioned behind guest host liquid crystals having homogenous (vertical) alignment (for example, Applied Physics Letters, Vol. 30, No. 12, pp 619, H. S. Cole and R. A. Kashnow (1977)). Under the condition of non-voltage, incident polarized light which is parallel to planely aligned liquid crystal molecules is absorbed into the dye on the way. Furthermore, linearly polarized light which is perpendicular to planely aligned liquid crystal molecules passes through a liquid crystal layer and is then converted to circularly polarized light by passage through the quarter wavelength plate 31 on the way, which is reflected by the reflection plate 32 and turns into circularly polarized light turning in the reverse direction. The linearly polarized light which is perpendicular to planely aligned liquid crystal molecules has its phase shifted at a 1/2 wavelength by passage through the quarter wavelength plate 31 on the way back, so that the linearly polarized light reenters the liquid crystal layer after being converted to linearly polarized light parallel to the liquid crystal molecules and then absorbed into the dichroic dye inside the liquid crystals. Therefore, the display becomes darker. On the other hand, when voltage is applied, the alignment will be vertical as shown in FIG. 5 (b), so that light absorption in the liquid crystal layer is reduced. As a result, the display becomes brighter.
However, the above-mentioned conventional system using the quarter wavelength plate was impractical because of its extremely low contrast. The reason for the low contrast is that retardation differs depending on the incident angle of the light, so that the light, which passed through the quarter wavelength plate back and forth, is not converted completely to linearly polarized light, and hence components arise which are not absorbed into dyes on the way back. In other words, polarized light in the light parallel to the liquid crystal molecular long axis is absorbed into a dichroic dye. Polarized light perpendicular to the liquid crystal molecular long axis passes through a liquid crystal layer and then enters a retardation film. A usual quarter wavelength plate is used as a retardation film, and n.sub.p is determined to be a principal refraction index in the slow axis direction forming an angle of 45.degree. with the polarized light, and n.sub.s is determined to be a principal refraction index in the fast axis direction perpendicular to this slow axis. Retardation of the polarized light which enters the plane including the liquid crystal molecular long axis and the substrate normal at an incident angle .theta. after passing through the liquid crystal layer can be described by a product of birefringence, which becomes smaller in accordance to the incident angle .theta., and a distance, which becomes greater in accordance to the incident angle .theta.. This can be shown as Formula 1 below.
Formula 1 EQU {n.sub.p n.sub.s /(n.sub.p.sup.2 sin.sup.2 .theta.+n.sub.s.sup.2 cos.sup.2 .theta.).sup.1/2 -n.sub.s }d/.lambda. cos .theta. PA0 Formula 2 EQU (n.sub.p -n.sub.s) d cos .theta./.lambda. PA0 Formula 3 EQU (n.sub.p -n.sub.s)d/.lambda.cos .theta. PA0 Formula 4 EQU n.sub.z =(n.sub.p +n.sub.s)/2
The retardation shown in Formula 1, which approximately can be shown as the following formula (Formula 2), is reduced in proportion to cos.theta. as the incident angle .theta. increases.
On the other hand, birefringence in the plane including the liquid crystal molecular short axis and the substrate normal is not dependent on the angle, so that the retardation can be shown as the following formula (Formula 3), and the retardation increases rapidly in inverse propertion to cos .theta. when the incident angle .theta. increases.
In this way, for example, even if retardation of the retardation film is determined to become .lambda./4 when the incident angle .theta. is 0.degree., the retardation of the retardation film changes greatly even when the incident angle .theta. declines about 30.degree.. Since light arises which is not absorbed into the dye on the way back, contrast is extremely deteriorated.
Furthermore, this system comprises liquid crystals with a homogeneous alignment, and steepness in threshold properties is insufficient, so that only a few pixels can perform matrix driving.
On the other hand, as a mode which is widely used at present, there is a super twisted nematic (STN) mode comprising the TN mode twisted even more. The STN mode also utilizes a retardation film. When STN liquid crystals are sandwiched between polarizers, birefringence color appears due to its short twist pitch. Then, according to the change in the amount of birefringence (retardation) caused by the voltage, the display changes its color. In order to eliminate coloration of the STN liquid crystals, the technology in using a retardation film made of a polymer has developed remarkably in recent years. Usually, for the purpose of eliminating the coloration resulting from birefringence of the STN liquid crystals, a polymer retardation film with a retardation of about 400 to 500 nm is used. However, when the retardation film used for eliminating the coloration has incident angle dependency, it causes the problem of display coloration etc. according to viewing angle, so that the following technique was proposed to solve this problem (for example, Y. Fujimura, T. Nagatuka, H. Yoshimi and T. Shimomura: SID' 91 Digest, 35.1 (1991)).
Against principal refractive indexes n.sub.p, n.sub.s inside the plane of the retardation film (n.sub.p &gt;n.sub.s, direction of n.sub.p is called a slow axis direction, and the direction of n.sub.s is referred to as fast axis direction), a principal refractive index n.sub.z in the thickness direction is usually equal to or smaller than n.sub.s. When a liquid-crystal panel is observed from the front face, n.sub.z does not play a part, but when the liquid-crysal panel is observed perspectively, the component of n.sub.z also contributes to the amount of birefringence. From the direction corresponding to Formula 3 mentioned above, namely, from the direction in the plane including the liquid crystal molecule short axis and the substrate normal, the birefringence index at the perspective view decreases by allowing n.sup.z to be larger than n.sub.s. Since the light passage is lengthened, the retardation change is reduced to counterbalance.
According to a simulation performed by Nagatuka et al. (supra), the incident angle dependency of retardation becomes minimum, provided that the relationship shown as Formula 4 below is satisfied.
A representative example of a retardation film which attains the relationship of the above-mentioned formula (Formula 4), is a three-dimentional reflective index control retardation film NRZ manufactured by Nitto Denko Corporation. In this retardation film NRZ, the principal refraction index n.sub.z in the thickness direction is also controlled by employing the stretching method of polycarbonate.
In addition, a retardation film satisfying Formula 4 mentioned above can be accomplished by connecting conventional retardation films having a positive and a negative birefringence index .DELTA.n.